Electron microscope including an electromagnetic electron energy analyzing lens

ABSTRACT

Apparatus for analyzing the energy of an electron beam in an electron microscope includes a cylindrical lens of the magnetic field type inserted additionally or in place of a conventional intermediate lens, and a slit member cooperating with said cylindrical lens, said cylindrical lens being excited under preselected conditions.

United States Patent 1 3,619,607

[ 72] Inventor Takeo Ichlnokawa [50] Field of Search 250/49.5 A, Yurigaoka Danchl 35-304, No. 716 v 49.5 D Takaishl, Kawasaki-sin, Kanagawa-ken, Japan [56] References Cited 21 1 Appl. N6. 49,575 UNITED STATES PATENTS 221 Filed June 24, 1970 3,256,433 6/1966 Watanabe et al. 250/49.5 A

[451 Q 'E 9, 1971 3,374,346 3/1968 Watanabe 250/49.5 A

Pmmy 1967 OTHER REFERENCES [31) 42/195334 Scanning Electron Diffraction with Energy Analysis,

Confinuafion of application sen No. Denbigh et al., Journal of Scientific Instruments Vol. 42 No. 5, 708,851, Feb. 2a, 1968, now abandoned. May

Primary Examiner-James W. Lawrence Assistant Examiner-C. E. Church An0mey-McGlew and Toren [54] ELECTRON MICROSCOPE INCLUDING AN fi ii ifi f z ELE 0N ENERGY ABSTRACT: Apparatus for analyzing the energy of an electron beam in an electron microscope includes a cylindrical 4 Chums 4 Drawing Figs' lens of the magnetic field type inserted additionally or in place [52] U.S. Cl i. 250/49.5 A, of a conventional intermediate lens, and a slit member 250/49.5D cooperating with said cylindrical lens. said cylindrical lens [5] Int. Cl H0 lj 37/26 being excited under preselected conditions.

PATENTEnunv 9 I97! 3.619.607

INVENTOR TAKE 0 ICHINoKhWQ BY m W ATTORNEY5 ELECTRON MICROSCOPE INCLUDING AN ELECTROMAGNETIC ELECTRON ENERGY ANALYZING LENS BRIEF SUMMARY OF THE INVENTION This is a continuation of 708,851 filed Feb. 28, 1968, now abandoned.

The present invention relates to an apparatus for analyzing the energy of an electron beam in an electron microscope or the like.

As is known, the analysis of the energy of an electron beam which has passed through a thin membrane or film of a sample, or of the beam which is scattered from a thick sample, is useful for many applications such as analysis of the image produced in an electron microscope, measurement of scattering and absorption of electrons in a solid material, identification of material of an extremely small quantity, etc.

In the apparatus which has been hitherto employed for the purposes above enumerated, a cylindrical lens of the electrostatic type is used as the means for analyzing the energy of the electron beam. However, the conventional apparatus of this kind has encountered many practical difficulties. For example, the electron beam analyzer lens occupies a great space due to its extremely large bulk and it is difiicult to install the lens in the column of the electron microscope. The maintenance and the operation is not only difficult, but also the operation of the electron microscope becomes difficult, owing to the use of the electrostatic lens. The apparatus can not be employed practically in applications where an electron accelerating voltage higher than 50 kv. is required.

The present invention contemplates the elimination of the difficulties to which the prior apparatus has been subject.

Accordingly, an important object of this invention is to provide a novel and improved apparatus for analyzing the energy of an electron beam in an electron microscope or the like.

Another object of this invention is to provide an apparatus for analyzing the energy of an electron beam, in which is employed under certain conditions a cylindrical analyzer lens of magnetic field type in place of the conventional electrostatic cylindrical lens.

According to this invention, the above object can be accomplishcd by providing an apparatus for analyzing the energy of electrons scattered from or having passed through the sample. said apparatus comprising an electromagnetic cylindrical lens interposed between an objective lens and a projection lens of the electron microscope and adapted to have applied thereto a strong excitation, and a slit member disposed in front of said cylindrical lens.

The other objects, features and advantages of this invention will become apparent from the following description taken with reference to the annexed drawings which show a preferable embodiment of this invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS FIGS. 1A and B schematically show electron optical systems of an embodiment according to this invention under different excitations.

FIG. 2 is a schematic perspective view of a cylindrical lens portion employed in the optical systems shown in FIGS. IA and B, and

FIG. 3 is a graph to illustrate the relation between lens strength (excitation strength) of the cylindrical lens and offaxial chromatic aberration.

DETAILED DESCRIPTION Now referring to FIGS. IA and B, the principle of this invention will be first explained.

Reference numeral 5 indicates an electron source which emits electron beam (e) of uniform energy. The electron beam e thus emitted is thereafter collimated by a condenser lens 6 to form a narrow collimated beam. A sample 7 is illuminated by this collimated electron beam and the image of the sample 7 is produced by an objective lens (8) on the plane of a slit member 4. The image thus produced is then formed by the lenses 3 and 9 on a fluorescent screen 11.

The velocity 83 electrons in the beam which has 64 through the sample 7 will be varied owing to the loss of energy such as caused by the inelastic scattering in the sample 7.

In order to analyze the energy of such electron beam, there is provided an analyzer lens 3 according to this invention, which cooperates with the slit member 4 to project that portion e, of the electron beam e which has passed through the slit 4 on the fluorescent screen 11 by way of the projecting lens 9, whereby the energy loss spectra, namely the image limited by the slit 4, are produced on the screen 11 depending on the variation of the amount of the energy loss.

The analyzer lens 3 of this invention is disposed in place of a conventional intermediate lens, and is composed of magnetic pole pieces 1, l and 2, 2 which are contoured and arranged so as to form a cylindrical configuration as a whole, as is clearly shown in FIG. 2. This analyzer lens 3 is magnetically excited by the power source of the conventional intermediate lens. The slit 4 is disposed normally to the refraction plane X of the electron beam.

FIG. 3 shows graphically the relation between the coefficient of off-axial chromatic aberration Cch of the magnetic cylindrical lens 3 and the lens strength or the excitation strength. In the coordinates of FIG. 3, the chromatic aberration coefficient Cch is taken along the ordinate, while taken along the abscissa is the value Nl/ V whereinV is the voltage for accelerating the electron beam, N is the number of turns of the exciting coil and l is current.

If the excitation parameter is represented by k which is defined as follows:

eHoa (in the circular symmetrical 8mVr lens or imaging lens) wherein e charge of an electron m mass of an electron H0 maximum strength of magnetic field along the lens axis a a half-width of the magnetic field, and

Vr accelerating voltage corrected by the relativistic factor, then the point B in the coordinates of FIG. 3 corresponds to the lens strength when k is three-fourths. This also corresponds to the lens strength when k is 3 in case of a circular symmetrical lens. When the excitation becomes stronger than that at the point A, the chromatic aberration is increased abruptly and reaches a maximum value at the point B, and its sign is reversed at the point B. The chromatic aberration is thereafter decreased progressively.

In the energy analysis, the analyzer lens has to be excited near the lens strength at B. FIG. 1A shows the corresponding optical system attained when the excitation strength is smaller than at the point A, while FIG. 18 shows the corresponding optical system when the excitation is stronger than that of the point B.

With the lens 3 employed in the strongly excited condition as above mentioned, the magnifications in the plane X, andin the plane Y perpendicular thereto, of the electron-microscopic image, become equal to each other by selecting the area of the image.

Furthennore, the astigmatic aberration of the image in the planes X and Y will remain substantially negligible in practice, since the depth of focus of the objective lens 8 which produces the image on the slit member 4, is very large due to the inherent nature of the electron microscope. Accordingly, if the slit member 4 is removed, an image similar to that obtained by a conventional electron microscope is projected on the fluorescent screen 11. When observing such image, if the slit member 4 is inserted in parallel to the plane Y by utilizing a suitable device such as the image selecting aperture device usually mounted on a conventional electron microscope, the image limited by the slit is projected on the fluorescent screen l I. This electron beam e is dispersed depending on the energy thereof, as shown in FIG. 2, because of the chromatic aberration of the lens 3. This operation is very simple and easily carried out as in the case of selected area difiraction. The above dispersion 6 of the electron beam can be mathematically expressed as follows:

wherein M is the magnification of the analyzer lens, r is distance from the axis of the magnetic cylindrical lens to the slit, Vr is the electron beam accelerating voltage corrected by the relativistic factor, and AV is loss of energy of the electron beam in the sample.

As is apparent from the above mathematical expression, if the distance r is increased, the dispersion of the spectrum will be increased. By selecting a suitable value of the excitation current, with INNV being selected within the range between the points A and B or greater than the point B so that chromatic aberration is sufficiently large, the maximum resolution can be attained.

in the above embodiment, pole pieces of a magnetic cylindrical lens are employed in place of those of the conventional circular lens when electron microscopic images are analyzed. However. in case of the energy analysis of electron diffraction patterns, it will be then necessary to dispose the magnetic cylindrical lens between the intermediate lens and the projection lens.

Furthermore, in the present invention, it is also possible to produce the electron-microscopic image by means of no loss electrons or loss electrons, using the energy selecting microscope method of the scanning type.

As is apparent from the above, the present invention provides an apparatus for, analyzing the energy of the electron beam, which can be easily constructed simply by replacing the pole pieces of a conventional circular lens by the inventive cylindrical lens or by inserting additionally the analyzer lens. It has been experimentally confirmed already that the apparatus according to this invention exhibits an energy resolution of l ev. at accelerating voltage higher than 50 kv. The apparatus of this invention can be successfully applied to an ultrahigh voltage electron microscopev In the foregoing, the present invention has been described with reference to preferably embodiments selected only by way of example. However, this invention is not restricted to such embodiment, and should be therefore broadly interpreted within the scope and spirit of the invention as defined in the claims.

What is claimed is:

1. In apparatus for analyzing the energy of an electron beam in an electron microscope and having an electron source, means for applying an accelerating voltage to electrons generated by said source, an objective lens, a projection lens and an intermediate lens between the objective and projection lenses; the improvement comprising an electromagnetically energized cylindrical lens; a slit member positioned in advance of said electromagnetically energized lens; and energizing means exciting said electromagnetically energized cylindrical lens with an excitation such that the square of the excitation parameter k is substantially three-fourths, which parameter k is given by the following expression lens.

3. In apparatus for analyzing the energy of an electron beam, the improvement claimed in claim 1, in which said electromagnetically energized cylindrical lens is positioned between said intermediate lens and said projection lens.

4. In apparatus for analyzing the energy of an electron beam, the improvement claimed in claim 1, in which said electromagnetically energized cylindrical lens includes two pairs of magnetizable pole pieces, each pole piece being semicylindrical and said pole pieces being arranged to form a substantially complete cylinder for passage of the electron beam between facing pole faces of the pole pieces of both pairs. 

2. In apparatus for analyzing the energy of an electron beam, the improvement claimed in claim 1, in which said electromagnetically energized cylindrical lens is said intermediate lens.
 3. In apparatus for analyzing the energy of an electron beam, the improvement claimed in claim 1, in which said electromagnetically energized cylindrical lens is positioned between said intermediate lens and said projection lens.
 4. In apparatus for analyzing the energy of an electron beam, the improvement claimed in claim 1, in which said electromagnetically energized cylindrical lens includes two pairs of magnetizable pole pieces, each pole piece being semicylindrical and said pole pieces being arranged to form a substantially complete cylinder for passage of the electron beam between facing pole faces of the pole pieces of both pairs. 